code: def __call__(self, batch_norm_func, inputs):
      if self.communication_backend_name == "nccl":
          return _NcclImpl(self.comm)
      else:
          return _MpiImpl(self.comm)
  
file_path: chainer/chainermn/functions/batch_normalization.py
imports: import chainer
  from chainer.backends import cuda
  from chainer.functions.normalization import batch_normalization
  import chainer.utils
label_desc: Use image node_id 2 for calling the MultiNodeBNImplSelector obj's underlying member method code with example usage: obj.__call__(batch_norm_func, inputs) and returns: _NcclImpl, _MpiImpl